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With version 3.0 (SVN 66386)
import pydoc
pydoc.gui()
gives
>>> Exception in thread Thread-1:
Traceback (most recent call last):
 File "/usr/local/lib/python3.0/threading.py", line 507, in 
_bootstrap_inner
 self.run()
 File "/usr/local/lib/python3.0/threading.py", line 462, in run
 self._target(*self._args, **self._kwargs)
 File "/usr/local/lib/python3.0/pydoc.py", line 1989, in serve
 DocServer(port, callback).serve_until_quit()
 File "/usr/local/lib/python3.0/pydoc.py", line 1971, in __init__
 self.base.__init__(self, self.address, self.handler)
 File "/usr/local/lib/python3.0/socketserver.py", line 401, in __init__
 self.server_activate()
 File "/usr/local/lib/python3.0/pydoc.py", line 1982, in 
server_activate
 if self.callback: self.callback(self)
 File "/usr/local/lib/python3.0/pydoc.py", line 2072, in ready
 text='Python documentation server at\n' + server.url)
 File "/usr/local/lib/python3.0/tkinter/__init__.py", line 1199, in 
configure
 return self._configure('configure', cnf, kw)
 File "/usr/local/lib/python3.0/tkinter/__init__.py", line 1190, in 
_configure
 self.tk.call(_flatten((self._w, cmd)) + self._options(cnf))
_tkinter.TclError: out of stack space (infinite loop?)

What tcl/tk version are you using ?
Also, can you put a "print(_flatten((self._w, cmd)) +
self._options(cnf))" before the line 1190 in
/usr/local/lib/python3.0/tkinter/__init__.py and tell what it shows when
you call pydoc.gui ?

I'm using Tcl/Tk 8.5.4 here
 print(":::")
 print(_flatten((self._w, cmd)) + self._options(cnf))
produces:
:::
('.3073836300', 'configure', '-yscrollcommand', '3077632332set')
:::
('.3073835564.3073835660', 'configure', '-text', 'Python documentation 
server at\nhttp://localhost:7464/')
>>> Exception in thread Thread-1:
Traceback (most recent call last):
 File "/usr/local/lib/python3.0/threading.py", line 507, in 
_bootstrap_inner
 self.run()
 File "/usr/local/lib/python3.0/threading.py", line 462, in run
 self._target(*self._args, **self._kwargs)
 File "/usr/local/lib/python3.0/pydoc.py", line 1989, in serve
 DocServer(port, callback).serve_until_quit()
 File "/usr/local/lib/python3.0/pydoc.py", line 1971, in __init__
 self.base.__init__(self, self.address, self.handler)
 File "/usr/local/lib/python3.0/socketserver.py", line 401, in __init__
 self.server_activate()
 File "/usr/local/lib/python3.0/pydoc.py", line 1982, in 
server_activate
 if self.callback: self.callback(self)
 File "/usr/local/lib/python3.0/pydoc.py", line 2072, in ready
 text='Python documentation server at\n' + server.url)
 File "/usr/local/lib/python3.0/tkinter/__init__.py", line 1201, in 
configure
 return self._configure('configure', cnf, kw)
 File "/usr/local/lib/python3.0/tkinter/__init__.py", line 1192, in 
_configure
 self.tk.call(_flatten((self._w, cmd)) + self._options(cnf))
_tkinter.TclError: out of stack space (infinite loop?)

Uhm, is it caused only by the combination of tk 8.5.4 and py3k, or does
the same happen with tk 8.5.4 and python-trunk ? What about tk 8.5.3 (or
some other of tk 8.5 series) or even tk 8.4.16 and {python-trunk, py3k} ? 
It would be good if you could test some other combination, specially
because I can't reproduce it here with other versions (but I'm compiling
8.5.4 here now).

Just reproduced the issue under python-trunk with tcl/tk 8.5.4 and 8.5.3

But it happens only if you don't build tcl/tk with --enable-threads =)

If you remove the widget.config calls in GUI.ready you will notice the
problem "goes away", but note that this method is called from another
thread while you have a non-thread-safe tcl/tk lib (I'm assuming you
didn't compile it with --enable-threads). So.. using multiple threads in
Python while Tcl is compiled without --enable-threads isn't supported at
all.
To reproduce the problem try this (change Tkinter to tkinter if py3k):
import threading
import Tkinter
lbl = Tkinter.Label(text="hi")
threading.Thread(target=lambda: lbl.configure(text="hi there")).start()
lbl.mainloop()
If your tcl/tk libs weren't compiled with --enable-threads this should
get you an "TclError: out of stack space (infinite loop?)". A
documentation note may be added somewhere, but nothing much else is
going to happen (that is what I believe at least).

Many thanks, that solved the problem.
Since the cause of the problem wasn't easy to find out
(for me, at least)
would be possible to check at import time if Tcl/Tk has been
configured with threads enabled?
Helmut.

The patch attached checks for that when an interpreter is created, not
really at import time but should be enough.
But my real concern is that tkinter thinks it will work properly when
Python is using threads and Tcl wasn't compiled with --enable-threads.
Maybe that was true some time ago or maybe in other platforms, but isn't
the case anymore.

Push updated patch for tip (3.3) head

New changeset d731dcda2611 by Martin v. Löwis in branch 'default':
Issue #3835: Refuse to use unthreaded Tcl in threaded Python.
http://hg.python.org/cpython/rev/d731dcda2611 

Thanks for the patch!

New changeset 7cbc48324938 by Andrew Svetlov in branch 'default':
Revert the patch for issue 3835 because failed on Windows buildbot
http://hg.python.org/cpython/rev/7cbc48324938 

Recall to 'open' state

Close as 'not a bug' because Guilherme's test to reproduce the bug from msg73304 works good at least for Linux. 
Windows version of Tcl/Tk compiled without threading support.
So current behavior is correct and should not to be fixed.